The present invention relates to an integrated circuit device for processing signals in general, and, more particularly, to an integrated circuit device that functions as a wideband mixer/detector for microwave to millimeter wave signals.
One of the most important, most widely deployed, and most well-known techniques in telecommunications is that of heterodyning. In accordance with heterodyning an information-bearing signal at baseband frequency is shifted into another frequency band, which is usually much higher than the baseband frequency. The ability to shift an information-bearing signal""s frequency is useful for two reasons. First, heterodyning enables multiple information-bearing signals at the same baseband frequency to be frequency-division multiplexed on a single communications channel. This is essential for ensuring the efficient bandwidth utilization of the communications channel. Second, heterodyning enables an information-bearing signal to be efficiently transmitted in the physical-limited passband of a communications channel (e.g., radio, optical fiber, etc.). In other words, heterodyning enables a frequency-shifted version of an information-bearing signal to be transmitted in the passband of a communications channel when the baseband version of the signal is outside of the passband.
FIG. 1 depicts a block diagram of a typical telecommunications system in the prior art that illustrates the process of heterodyning. In transmitter 101, an information-bearing signal at one frequency, F1, is used to modulate a carrier signal at a second frequency, F2, which results in the generation of a lower sideband signal at a third frequency, |F1xe2x88x92F2|, and an upper sideband at a fourth frequency |F1+F2|. Typically, the frequency of the carrier signal, F2, is much larger than F1 and is carefully chosen so that the frequency of both the upper and lower sidebands are within the passband of communications channel 102. The modulated carrier signal is then transmitted via communications channel 102, which can be either wireless or wireline, to receiver 103. In receiver 103, the modulated carrier signal is demodulated to recover the information-bearing signal.
Interestingly, both the process of modulating the carrier signal (at the transmitter) and the inverse process of demodulating the modulated carrier signal (at the receiver) can be performed with the same device operating in the same fashion. In other words, the process of modulating the carrier signal and the inverse process of demodulating the modulated carrier signal are involutory (i.e., the function f(x) is involutory if, and only if, f(f(x))=x).
When the device is used for modulating, it is often called a xe2x80x9cmixerxe2x80x9d because the modulation of the carrier is often conceived of as mixing the information-bearing signal with the carrier. When the device is used for demodulating, it is often called a xe2x80x9cdetectorxe2x80x9d because the device effectively detects the information-bearing signal within the modulated carrier signal. In contrast, some practitioners call the device a mixer when it is both modulating a carrier and when it is demodulating a carrier. For the purposes of this specification, the device itself, out-of-context, is called a xe2x80x9cmixer/detector.xe2x80x9d
FIG. 2 depicts the process performed by a xe2x80x9cmixer/detectorxe2x80x9d when the device is used for both modulating and demodulating. Two signals, one at frequency F1 and the second at frequency F2 are fed into the device, which because of its inherent physical properties creates four signals: F1, F2, F1xe2x88x92F2, and F1+F2. When the device is used for mixing (i.e., modulation), then the signal F1 is filtered out. In contrast, when the device is used for detecting (i.e., demodulation), the process is the same. To illustrate this, let the arriving modulated carrier at the receiver have a frequency of F3=F1xe2x88x92F2 and the second input signal to the detector be the carrier signal at frequency F2. Then the output of the mixer/detector are the four signals: F2, F3, F2xe2x88x92F3, and F2+F3. Substituting for F3=F1xe2x88x92F2, the output signals are: F1, F2, F1xe2x88x92F2, and F1+F2. The three signals F2, F1xe2x88x92F2, and F1+F2 can be filter out with a bandpass filter to leave the information-bearing signal at F1.
A mixer/detector has several operating characteristics:
1. it operates over a wide range of frequencies;
2. it has a wide modulation bandwidth;
3. it requires a low local oscillator power;
4. it has a high conversion gain;
5. it has a low noise figure;
6. it has resistive electrical characteristics; and
7. it operates at ambient or relatively high temperatures.
There are several mixer/detectors in the prior art that are responsive to millimeter and submillimeter waves. These include Schottky diodes, superconducting insulating superconductor (xe2x80x9cSISxe2x80x9d) junction mixer/detectors, lattice-cooled bolometric mixer/detectors, and diffusion-cooled bolometric mixer/detectors. K. S. Yngvesson, Ultrafast Two-Dimension Electron Gas Detector and Mixer for Terahertz Radiation, Applied Physics Letters, Vol. 76, No. 6, pp. 777-779 (Feb. 7, 2000) provides a discussion of diffusion-cooled bolometric mixers.
For the purposes of this specification, a xe2x80x9cdiffusion-cooled bolometric mixer/detectorxe2x80x9d is defined as a mixer/detector whose predominant method of cooling electrons is by transferring, through diffusion, the hot electrons into the mixer/detector""s contacts. For the purposes of this specification, a xe2x80x9clattice-cooled bolometric mixer/detectorxe2x80x9d is defined as a mixer/detector whose predominant method of cooling electrons is by transferring the energy in the hot electrons into phonons that vibrate the lattice of the mixer/detector. Because all of the mixer/detectors in the prior art exhibit at least some disadvantages, the need exists for a mixer/detector that overcomes some of these disadvantages.
The present invention provides a structure that is useable as a mixer or as a detector or both. In particular, the illustrative embodiment of the present invention is a velocity-cooled hot-electron bolometric mixer/detector, which for the purposes of this specification is defined as a mixer/detector whose predominant method of cooling electrons is by transferring without scattering the hot electrons into the mixer/detector""s contacts.
In one aspect, the invention comprises: a compound semiconductor substrate comprising at least one element from group 3 of the periodic table of the elements and at least one element from group 5 of the periodic table of the elements; a first undoped compound semiconductor layer deposited on the compound semiconductor substrate, wherein the first undoped compound semiconductor comprises at least one element from group 3 of the periodic table of the elements and at least one element from group 5 of the periodic table of the elements; a second undoped compound semiconductor layer deposited on the first undoped compound semiconductor layer, wherein the second undoped compound semiconductor comprises at least one element from group 3 of the periodic table of the elements and at least one element from group 5 of the periodic table of the elements, and wherein the second undoped compound semiconductor layer has a wider band gap than the first undoped compound semiconductor layer; and a third layer deposited on the second undoped compound semiconductor layer, wherein the third layer comprises at least one element from group 4 of the periodic table of the elements.